In order to search a document from a document database, a user inputs a search option such as a search term to be used for searching. Selecting a useful search term that would lead to a document that satisfies the user's need has been difficult as the search term that is too general may return a large number of documents and the search term that is too specific may return a small number of documents.
Especially when a plurality of search terms are used for searching, it has been difficult to select a combination of search terms that would lead to a sufficient number of documents satisfying the user's need as a large number of combinations of search terms are available.
Further, when a large number of documents are obtained as a search result, it has been difficult to analyze the search result to select a small number of documents satisfying the user's need.